1. Field of the Invention
This invention relates to an image forming apparatus such as a laser printer capable of two sided printing, that is forming images on the front and back surfaces of a sheet, and particularly to an image forming apparatus to the main body of which is detachably mountable two-sided copy a conveying portion (duplexing portion) for reversing the surfaces of a sheet having an image formed on one surface thereof and conveying it to an image forming portion.
2. Related Background Art
Some of conventional image forming apparatuses are such that a sheet having an image formed on one surface thereof by an image forming portion is reversed and is again conveyed to the image forming portion so that an image may be formed on the back surface of the sheet.
In an image recording apparatus such as a laser printer which is an example of the conventional image forming apparatuses, a two-sided copy conveying portion having a two-sided copy conveying path for reversing the front surface and back surface of a sheet and conveying the sheet to an image forming portion is optionally mounted on a relatively bulky, high-speed and high-cost apparatus so that images may be recorded on the both surfaces of the sheet.
Now, in recent years, even in compact, low-speed and low-cost laser printers, there is a growing trend toward including the function of recording images on the both surfaces of a sheet for the saving of natural resources, but when a two-sided copy conveying portion is to be optionally mounted on such a compact, low-speed and low-cost image recording apparatus, a two-sided copy conveying portion of low cost becomes requisite as a matter of course.
Such two-sided copy conveying portion, however, requires various sensors such as a jam detecting sensor for detecting, for example, the occurrence of jam in a two-sided copy conveying path and a jam clearance detecting sensor for detecting jam clearance being carried out, and the duplex conveying portion becomes high in cost.
Also, in the conventional image recording apparatus, when such a two-sided copy conveying portion is to be mounted, detecting means for detecting whether the two-sided copy conveying portion has been mounted must be newly provided and thus, the total cost of the apparatus becomes high.